This invention relates to synchronous power transmission systems and toothed belts therefor. Synchronous drive systems of this type are used extensively in industry and particularly in the automotive industry for driving cam shafts and auxiliaries on engines.
More particularly, the present invention is concerned with a toothed belt having teeth whose profiles are defined by curvilinear lines and with a combination of such belt with a pulley having grooves and teeth which are generally trapezoidal in cross section. In prior art systems of this type as for example in systems which comprise a belt having teeth of trapezoidal cross-section and a pulley having grooves of trapezoidal cross-section as specified, for example, in the "Timing Belt Drive Engineering Handbook" by R. Y. Case, McGraw Hill Book Co. Inc. 1954, the belt is likely to generate unpleasant sounds when its teeth engage the pulley grooves or disengage therefrom. Depending on the shape of the belt teeth and/or the engaged relationship between the belt teeth and the pulley, engagement and disengagement causes vibration of the belt. In the event a driving shaft, a driven shaft, an idler shaft, or any other element surrounding the pulley has a natural frequency which is the same or a multiple of the frequency of the belt vibration, resonance occurs and generates unpleasant sounds. When such a device is employed in an automobile engine or in a similar engine, it is particularly true that resonance often occurs in some portion of the speed range of the engine and generates unpleasant noise.
In order to overcome these disadvantages, there has been developed a drive system in which the belt teeth have a height greater than the depth of the pulley grooves and are adapted to be compressibly engaged therein, as disclosed in U.S. Pat. No. 4,037,485. According to this system, the tooth tips of the belt teeth are compressibly deformed by the bottom surface of the pulley grooves so that they decrease in height. The belt teeth, according to that system, are complementary in shape to the pulley grooves and contact the entire surface thereof. The slanted flanks of the belt teeth contacting the corresponding slanted flanks of the pulley grooves have a surface area enlarged to the maximum possible extent, so that a low contact pressure may be maintained therebetween in an effort to reduce all other tooth deformation. Accordingly, the belt teeth have only a limited degree of freedom for compressive deformation in the pulley grooves. Such belt teeth fail to sufficiently absorb impact and thus substantial noise is generated when each belt tooth engages and/or disengages from a pulley groove. Some of that noise is of the type which is classified as unpleasant noise by a majority of the car buying public.
According to another known synchronous drive system, that described in U.K. Patent Application GB No. 2084688 (Mitsubishi) a standard trapezoidal pulley is used in conjunction with a belt whose teeth, in cross-section, are defined by straight lines which occupy 60% to 95% of the total tooth height and a tooth crest formed by an arc whose center of curvature is on the centerline of the tooth. Here again the tooth flanks are flat trapezoidal shapes resulting in problems similar to those described above. This belt relies upon "bottoming", as described above, in order to achieve some impact absorption. However, manufacturing tolerances of both the belts and the pulleys may result in the belt teeth having a height shorter than the depth of the pulley groove, in which event the teeth will not bottom and will act as conventional trapezoidal teeth.
Another known belt is that described in U.S. Pat. No. 4,371,363 to Cicognani, et al. The belt teeth according to this patent have arcuate convex flanks defined by the relationship that the trigonometric tangent of the curved flank increases linearly from the tip end of the tooth toward the base thereof. This belt also does not provide a satisfactory solution to the problem of unpleasant noise generated as a result of certain belt vibration when in driving or driven engagement with a pulley.
Moreover, many other types of toothed belts are known with teeth having a curved profile that is formed by combining arcs of a circle with rectilineal portions or with involute profiles, but even those known belts vibrate when employed in transmission systems of the type referred to above. No successful means for solving the problem of vibration resulting in unpleasant noise has been found up to this time.
It is therefore an object of the present invention to improve the shape of the belt teeth such that the impact of meshing is absorbed as the belt teeth mesh with their mating pulley grooves resulting not only in reduced belt vibration but less generation of unpleasant noise.
It is also an object of the invention to provide an improved power transmission system which minimizes the generation of unpleasant sounds due to vibration resulting from meshing between the belt teeth and the pulley grooves.
Industrial manufacturers, particularly automobile manufacturers have experts, sometimes known as "noise engineers" who are able to distinguish the different forms of noise and are able to agree on the quality of noise which is acceptable to the buying public.
According to the present invention there is provided an endless flexible power transmission belt having a plurality of teeth separated by grooves. Each tooth in longitudinal cross-section is symmetrical about a tooth center line and includes a root portion, a tip portion and opposite convex curvilinear flank portions extending between said tip portion and said root portion. The root portion is defined by a first and a second arc each approximating the arc of a circle on opposite sides of the tooth center line and extending into the corresponding flank portions of the tooth and each said groove has a base portion on the belt land line connecting said first arc of one tooth with said second arc of the next consecutive tooth, said tooth having a height measured between the extremity of said tooth tip portion and said belt land line and has a width measured between said opposite flank portions along a width line parallel to said belt land line at a distance therefrom of approximately 40% to 50% of said tooth height. Each of said opposite convex curvilinear flanks is formed such that when an imaginary line passing through the point at which the corresponding one of said first and second arcs intersect the flank portion in question, drawn at an angle of between 19.degree. and 25.degree. with respect to said tooth centerline and intersecting said flank, it will intersect said width line at a point located between 94% and 98% of the distance from said centerline to the surface of the convex curvilinear flank as measured along said width line.
According to the present invention there is also provided a power transmission system including a flexible drive belt having a series of spaced teeth which are substantially curvilinear in longitudinal cross-section, in meshing engagement with a pulley also having a plurality of teeth separated by grooves, each pulley groove having a longitudinal cross-sectional contour that is substantially trapezoidal in shape.
The curvilinear belt tooth flank according to the present invention is preferably shaped such that the initial contact with the pulley groove flank occurs at a point on the profile of the belt tooth flank spaced from the belt land line a distance equal to approximately 45% of the belt tooth height and a point on the pulley groove flank spaced from the periphery of the pulley a radial distance equal to approximately 45% of the pulley groove depth. Furthermore, as the belt tooth according to the present invention continues to mesh with the mating pulley groove the curvilinear belt tooth flank is compressively deformed to the extent that the one-half tooth width as measured along said width line is reduced by an amount equal to approximately 4% of said one-half tooth width.
The initial point contact between the belt tooth and pulley groove flanks in combination with the continuing belt tooth flank compressive deformation during meshing is believed to have a belt vibration dampening effect resulting in a lower incidence of generating unpleasant sound.